difficulty on the game solo vs squad?

Full premade squads are preferrably matched with other full squads. I don't think this has ever been stated officially, but I'm 99% sure such algorithm is in place. No idea what it does if there are not enough players for either squad/solo games - probably ditches the rule out the window and mixes everyone together, but anyway, you should still encounter more full premades if you start in one yourself.

That may or may not feel different sometimes (grouping up does not increase player's skill, so facing more premades doesn't mean it will be harder or easier).